[Qemu-devel] [PATCH 20/56] nbd: Don't trim unrequested bytes

[Michael Roth]

From: Eric Blake <address@hidden>

Similar to commit df7b97ff, we are mishandling clients that
give an unaligned NBD_CMD_TRIM request, and potentially
trimming bytes that occur before their request; which in turn
can cause potential unintended data loss (unlikely in
practice, since most clients are sane and issue aligned trim
requests).  However, while we fixed read and write by switching
to the byte interfaces of blk_, we don't yet have a byte
interface for discard.  On the other hand, trim is advisory, so
rounding the user's request to simply ignore the first and last
unaligned sectors (or the entire request, if it is sub-sector
in length) is just fine.

CC: address@hidden
Signed-off-by: Eric Blake <address@hidden>
Message-Id: <address@hidden>
Signed-off-by: Paolo Bonzini <address@hidden>
(cherry picked from commit 353ab969730742b7392414d62f4ba9632e8cf22c)
Signed-off-by: Michael Roth <address@hidden>
---
 nbd/server.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c
index 2184c64..cc4bda3 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1153,12 +1153,20 @@ static void nbd_trip(void *opaque)
         break;
     case NBD_CMD_TRIM:
         TRACE("Request type is TRIM");
-        ret = blk_co_discard(exp->blk, (request.from + exp->dev_offset)
-                                       / BDRV_SECTOR_SIZE,
-                             request.len / BDRV_SECTOR_SIZE);
-        if (ret < 0) {
-            LOG("discard failed");
-            reply.error = -ret;
+        /* Ignore unaligned head or tail, until block layer adds byte
+         * interface */
+        if (request.len >= BDRV_SECTOR_SIZE) {
+            request.len -= (request.from + request.len) % BDRV_SECTOR_SIZE;
+            ret = blk_co_discard(exp->blk,
+                                 DIV_ROUND_UP(request.from + exp->dev_offset,
+                                              BDRV_SECTOR_SIZE),
+                                 request.len / BDRV_SECTOR_SIZE);
+            if (ret < 0) {
+                LOG("discard failed");
+                reply.error = -ret;
+            }
+        } else {
+            TRACE("trim request too small, ignoring");
         }
         if (nbd_co_send_reply(req, &reply, 0) < 0) {
             goto out;
-- 
1.9.1